The invention relates to a method and a device for injecting a fuel/liquid mixture into the combustion chamber of a burner.
In the firing of combustion chambers which are used for energy generation and, particularly in combination with following gas turbine plants, make it possible to convert fossil fuels into electric energy, there is the constant endeavour to optimize the combustion operation in terms of the complete combustion of the fuels used and the reduction of emissions occurring due to combustion.
Thus, for example, incomplete combustion of the fuels introduced into the combustion chamber leads to increased NOx emission values which can be decisively reduced by admixing the fuel with water.
During the operation of gas turbine plants, the burner of which is operated with a fuel/water mixture in order to avoid increased NOx emission values, it becomes apparent that intensive low-frequency vibrations are established within the burner unit, the amplitude of these vibrations rising with an increasing water fraction in the fuel/water mixture. A plurality of discrete resonant vibrations with different frequencies are formed, depending on the size and shape of the burner unit.
Such pronounced vibration phenomena at low frequencies seem, with a great degree of certainty, to be caused by diffraction effects of entropy waves at the turbine inlet which follows the combustion chamber in the direction of flow. Entropy waves occur primarily when the temperature ratio over the entire flame within the combustion chamber is markedly higher than 2. For this reason, vibrations of this kind are often established between the ignition event and the idling mode of the burner, especially since the air flowing into the combustion chamber has a relatively low temperature, as compared with the flame temperature, so that the temperature ratio over the entire flame is very high.
In order to suppress the low-frequency vibrations which are formed, relevant modifications could be made to the combustion chamber and its connection to the turbine inlet orifice, these modifications making it possible to vary, and appropriately modulate, the resonant behaviour of this volume region. For example, it is appropriate, for this purpose, to provide suitable bypass lines in the combustion chamber region or else in the high pressure turbine region, these bypass lines making it possible to achieve controlled vibration damping. However, modifications of this kind would necessitate expensive reconstruction measures on existing gas turbine plants, but such measures are unattractive due to the associated high cost outlay and a long-term operational shutdown of the plant.
The objection which the invention is based is to develop a method and a device for injecting a fuel/liquid mixture into the combustion chamber of a burner with an atomizer nozzle, in such a way that the occurrence of intensive low-frequency vibrations within the combustion chamber can be largely eliminated, without the need for cost-intensive reconstruction measures on existing plant components, in particular in completed gas turbine plants.
The invention is based on the knowledge that the resonant behaviour of low-frequency vibrations caused within the combustion chamber by the formation of entropy waves is determined essentially by a duration  which corresponds to the time interval between the outflow of the fuel/liquid mixture from the atomizer nozzle and the ignition event within the combustion chamber.
Investigations on a burner model setup, which provides, upstream of entry into the combustion chamber, an atomizer nozzle suitable for supplying a fuel/liquid mixture and the resonant behaviour of which was measured with the aid of a loudspeaker system setting the burner in vibration, showed that, depending on the acoustic framework conditions which result, in particular, from the geometry of the burner shape and burner volume, definite resonant frequencies are formed, one of which corresponded approximately to the Helmholtz resonant frequency of the acoustic system. Measurements of the sound energy absorbed by the burner system and provided by the loudspeaker system indicated, in the case of a resonant vibration behaviour, the burner system absorbs the introduced sound energy to the greatest extent, if  assumes very low values, near zero, or values near the reciprocal value of the corresponding resonant frequency. In other words, it is possible, by an appropriate choice of the duration , to excite the modes of the entropy waves particularly sharply. Conversely, this also means that, by a suitable choice of the parameter , a variable was found, by the suitable choice of which operating states of a burner can be set, in which the excitation of entropy waves causing the formation of the low-frequency vibrations can be largely suppressed. The parameter , which indicates the duration between the injection of the fuel/water mixture and the combustion event, thus plays a decisive role in suppressing the low-frequency vibrations.
Further investigations showed that the duration  rises with an increasing liquid fraction which is admixed with the fuel. However, so that, despite the existing dependence on the quantity of the liquid admixed with the fuel, the duration  can be set at suitable values at which the vibrational excitation of the entire burner system can, if possible, be suppressed, it was recognized, according to the invention, that the duration  can be reduced by means of controlled swirling of the liquid, water in the above case, to be admixed with the fuel, so that, by the controlled introduction of vortices into the incoming liquid stream before or during intermixing with the fuel, the duration  can be set essentially as desired and independently of the liquid fraction within the fuel/liquid mixture.
According to the invention, a method for injecting a fuel/liquid mixture into the combustion chamber of a burner, with an atomizer nozzle and with at least one liquid delivery region, at least one fuel delivery region separated from the latter and at least one volume region which is provided upstream of the atomizer nozzle orifice and in which the liquid is mixed with the fuel, is designed in such a way that the liquid undergoes swirling before or during intermixing with the fuel.
Since, as mentioned above, the duration  increases with an increasing liquid fraction in the fuel/liquid mixture, it is possible to compensate the increase of  by the increased formation of vortices in the incoming liquid stream. By the appropriate introduction of vortices into the incoming liquid stream or into the liquid flow to be intermixed with the fuel, the ignition time or the duration  can be set individually in dependence on the liquid fraction to be admixed, so that the formation of entropy waves can, if possible, be prevented.
A device according to the invention for injecting a fuel/liquid mixture into the combustion chamber of a burner, with an atomizer nozzle, for carrying out the abovementioned method according to the invention is designed in that a vortex-generating element is provided in at least one liquid delivery region or the liquid delivery region is arranged in such a way that the liquid flow is swirled before or during intermixing with the fuel.
The degree of swirling can be set by means of appropriate design measures within the liquid delivery region, for example by the inclination of deflecting plates relative to the direction of flow of the liquid or by appropriately made passage orifices between the liquid delivery region and the fuel delivery region.
Atomizer nozzles known per se have a delivery region axially relative to the nozzle outlet orifice for the fuel to be atomized. After the fuel, preferably fuel oil, has passed out through the nozzle orifice, said fuel is atomized into very fine fuel droplets. In the device according to the invention, before the fuel flows out of the nozzle outlet orifice, liquid, preferably water, is admixed with the fuel, said water being intermixed in swirled form with the fuel before the latter flows out of the nozzle outlet orifice.
For this purpose, it is particularly suitable to have passage orifices between the liquid delivery region and the fuel delivery region, the liquid being introduced through said orifices perpendicularly to the flow of the fuel. In this way, the liquid entering the fuel flow assumes an approximately spirally spreading vortex, by means of which the liquid is intermixed with the fuel.
In an alternative nozzle arrangement, a coaxial liquid flow is provided, separated by an intermediate wall, around the fuel flow directed axially relative to the nozzle outlet orifice, said liquid flow being changed by means of vortex-generating elements within the liquid delivery region into a vortex rotating coaxially around the fuel delivery region. Just upstream of the atomizer nozzle outlet orifice, the two flows collide and pass in swirled form to the nozzle outlet orifice.
The vortex intensity within the liquid flow can be appropriately determined, depending on the fraction of liquid to be admixed with the fuel.